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CO₂ permeability and bicarbonate transport in microperfused interlobular ducts isolated from guinea-pig pancreas

H. Ishiguro, S. Naruse, M. Kitagawa, A. Suzuki, A. Yamamoto, T. Hayakawa, R. M. Case * and M. C. Steward *

1. Permeabilities of the luminal and basolateral membranes of pancreatic duct cells to CO₂ and HCO₃^- were examined in interlobular duct segments isolated from guinea-pig pancreas. Intracellular pH (pH_i) was measured by microfluorometry in unstimulated, microperfused ducts loaded with the pH-sensitive fluoroprobe 2'7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF).

2. When HCO₃^-/CO₂ was admitted to the bath, pH_i decreased transiently as a result of CO₂ diffusion and then increased to a higher value as a result of HCO₃^- uptake across the basolateral membrane by Na⁺-HCO₃^- cotransport.

3. When HCO₃^-/CO₂ was admitted to the lumen, pH_i again decreased but no subsequent increase was observed, indicating that the luminal membrane was permeable to CO₂ but did not allow HCO₃^- entry to the cells from the lumen. Only when the luminal HCO₃^- concentration was raised above 125 mM was HCO₃^- entry detected. The same was true of duct cells stimulated with forskolin.

4. Recovery of pH_i from an acid load, induced by exposure to an NH₄⁺ pulse, was dependent on basolateral but not luminal Na⁺ and could be blocked by basolateral application of methylisobutylamiloride and H₂DIDS. This indicates that the Na⁺-H⁺ exchangers and Na⁺-HCO₃^- cotransporters are located exclusively at the basolateral membrane.

5. In the presence of HCO₃^-/CO₂, substitution of basolateral Cl^- with glucuronate caused larger increases in pH_i than substitution of luminal Cl^-. This suggests that the anion exchanger activity in the basolateral membrane is greater than that in the luminal membrane.

6. We conclude that the luminal and basolateral membranes are both freely permeable to CO₂, but while the basolateral membrane has both uptake and efflux pathways for HCO₃^-, the luminal membrane presents a significant barrier to the re-entry of secreted HCO₃^-, largely through the inhibition of the luminal anion exchanger by high luminal HCO₃^- concentrations.

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